In the development of dielectric fluids to replace polychlorinated biphenyls for transformers and capacitors, it was found feasible to develop separately fire resistant fluids specifically for transformers, and other fluids for capacitors that are not as fire resistant. Polychlorinated biphenyls (PCB's) were used in both applications because they possess good dielectric and physical properties and high fire resistance, especially where safety is important as it is with indoor distribution transformers. But their use was discontinued because the PCB's posed possible ecological and health problems. The fluids available now for capacitors are highly aromatic for good corona resistance, such as Wemcol dielectric fluid (isopropylbiphenyl), or else have a relatively high dielectric constant, such as the phthalate esters. They are not highly fire resistant, but this compromise was reluctantly allowed with the justification that capacitors contain relatively small amounts of fluid and do not present a serious fire hazard. For transformers, which have to satisfy more stringent fire safety requirements, two fluids are available--50 centistoke dimethyl silicone, and a high molecular weight hydrocarbon. These have Cleveland Open Cup Test fire points that are above 300.degree. C., which is the minimum level presently acceptable for fire resistant electrically insulating fluids. But their dielectric properties are relatively poor for use in capacitors, since they have low dielectric constants and poor corona resistance. It is obvious that it would be desirable to have a capacitor dielectric fluid available that is both fire resistant and has satisfactory electrical properties.
The difficulty in finding or developing such a fluid can be seen by considering the often conflicting characteristics needed or desired. Practical and economic considerations require that the fluid be organic. It should possess polarity or aromaticity, for respectively high dielectric constants in low voltage capacitors or corona resistance in high voltage capacitors, and should have low conductivity or dissipation factor. It should have low vapor pressure, about 5 Torr or less at 300.degree. C., to satisfy the fire resistance requirement; this usually means the molecules must contain at least about 35 carbon atoms. The fluid must be a liquid (i.e., non-crystalline) at at least -30.degree. C., and this is helped by the presence of aliphatic groups. It should easily impregnate a dielectric winding containing film, for example film-paper or all film, which in general seems to require relatively low viscosity and small molecular size. But a molecular characteristic that is responsible for the attainment of one of these properties can harm one or more of the others. Aromaticity, polarity and high molecular weight promote solidification at moderate temperatures. Also, high molecular weight and concomitant high viscosity impede impregnation of film. Polarity causes the conductivity to increase. Aliphatic groups diminish corona resistance. In view of this, finding material that is both fire resistant and has good enough dielectric properties for capacitors is a matter of chance and cannot be predicted.